There has been a recently renewed interest in vehicle propulsion systems in which stored electrical energy is the primary or sole source of energy. Of course, one immediately thinks of batteries as the most likely such energy source. Unfortunately, batteries are generally limited in their capability for meeting transient high power density requirements. Accordingly, the performance requirements for batteries to be used in electric vehicle propulsion systems can be significantly ameliorated by also using capacitors as supplemental and more flexible energy storage devices to supplement battery power when transient high power densities are required, thereby permitting load leveling. Of course in vehicle propulsion systems, as well as other portable pulse power device applications, the weight and volume of the power source, batteries and capacitors must be minimized to improve operating efficiencies and reduce the time between required access to stationary power supplies for recharging batteries and the like. Accordingly, conventional capacitors which are designed without any significant concerns for weight and volume efficiencies are unacceptable for such applications. Even recent technological advances in capacitors such as aluminum electrolytic capacitors, tantalum capacitors and ceramic capacitors do not satisfy the extremely high energy density and power density requirements as well as voltage flexibility that are needed in modern high pulse power applications. Attempts at meeting such special energy storage applications have resulted in the development of what are known as ultracapacitors and supercapacitors. Ultracapacitors and supercapacitors are two types of electric double-layer capacitors, utilizing ultrathin porous electrodes which in turn encapsulate small quantities of electrolyte. There are two distinctions between the two. First, supercapacitors display enhanced charge storage due to Faradaic charge transfer in addition to the double layer effect. Second, the cathode material differs in the two classes of devices. Ultracapacitors which are presently available commercially contain two carbon electrodes composed of high surface area carbon mixed with a binder. Each electrode is supported by a discrete metal film or grid which is several microns thick and which serves as a current collector. The cathode and anode are separated by an insulating cell separator. The entire structure is permeable and filled with an aqueous or non-aqueous electrolyte mixture. Supercapacitors, which are currently available only in the laboratory, are structurally analogous except that the carbon electrode of the ultracapacitor is replaced by an electrode composed of transition metal oxide or hydrous oxide. For example, supercapacitors known in the art have used electrodes composed of one or more oxides of ruthenium, tantalum, rhodium, iridium, cobalt, nickel, molybdenum, tungsten or vanadium deposited on a metal foil. The electrolyte may be acidic, basic or neutral, such as sulfuric acid, potassium hydroxide or sodium sulfate. Supercapacitors typically employ stacks of laminated electrodes consisting of a separator between the electrodes. Ion permeable membranes have been used as separators. The particular configuration depends upon the application. Unfortunately ultracapacitors which are currently commercially available have low volumetric and gravimetric energy densities, as well as low cell voltages. Supercapacitors are not yet commercially available at all. Neither existing ultracapacitors nor existing supercapacitors meet the requirements for incorporation into electric vehicle propulsion systems.
A recent search conducted in the U.S. Patent and Trademark Office indicates the following U.S. Pat. Nos. as being relevant in varying degrees to the invention disclosed herein.
4,179,812 Rayno PA1 4,313,084 Hosokawa et al PA1 4,323,950 Bernard PA1 4,327,400 Muranaka et al PA1 4,442,473 Holtzman et al PA1 4,480,286 Whitman et al PA1 4,480,290 Constanti et al PA1 4,538,208 Shedigian PA1 4,670,814 Matsui et al PA1 4,731,705 Velasco et al PA1 4,734,821 Morimoto et al PA1 4,768,130 Bernard et al PA1 5,047,899 Bruder PA1 5,055,975 Behrend PA1 5,079,674 Malaspina PA1 5,086,374 MacFarlane et al
Of the foregoing patents, the following provide the most relevant disclosure with respect to the invention claimed herein.
U.S. Pat. No. 4,313,084, to Hosokawa et al is directed to several embodiments of laminated capacitor structures comprising a plurality of double layer unit capacitor cells, each having a high withstand voltage, with voltage equalization between cells provided by a pattern of resistors on a flexible sheet wrapped around the laminated capacitor assembly before being accommodated in an outer casing. The unit cell of the double layer capacitor comprises paste electrodes of activated carbon in an electrolyte solution sandwiched between disc conductive separators with a porous separator between the electrodes enclosed by a ring-shaped non-conductive gasket. The assembly of seven unit cells in series with terminal leads of resistors clamped between the unit cells is shown. A four-cell assembly is also shown.
U.S. Pat. No. 5,079,674, to Malaspina is directed to a method of fabricating a supercapacitor electrode by absorbing metal oxides onto a high surface area material such as activated carbon. Soluble metal salts of ruthenium, tantalum, rhodium, iridium, cobalt, nickel, molybdenum, tungsten, or vanadium are adsorbed onto the conductive carbon matrix. The salts are converted to oxides and the matrix is mixed with a resin, formed into sheets and then laminated onto an absorbent separator. A second electrode sheet can be laminated onto the opposing side of the separator. The assembly can be cut to size and stacked to form supercapacitors. The stack is connected to leads placed in a suitable container and activated by adding a liquid electrolyte.
U.S. Pat. No. 5,086,374, to MacFarlane et al is directed to an aprotic electrolyte for an electrolytic capacitor, double capacitor, or battery comprising at least one salt, one acid, and an aprotic solvent. The capacitor anode and cathode can be any suitable metal in the form of a foil, or formed in situ by painting, sputtering, evaporating, or depositing on the spacer, or for low voltage, low frequency energy storage, a double layer utilizing a mass of conductive carbon particles can be formed on the spacer. The salt is selected from the group consisting of alkali metal salts, transition metal salts, ammonium and ammonium derivative salts, zinc salts, cadmium salts, mercury salts, lead salts, bismuth salts, and thallium salts, or may be partial esters of at least one of boric sulfuric and phosphoric acid reacted with compounds containing an alcohol group or alternately may be an alkali metal tetrafluoroborate. The acid may be selected from the group consisting of perchloric acid, tetrafluoroboric acid, thiocyanic acid, trifluoromethanesulphonic acid, and haloid acids. The aprotic solvent in the electrolyte is made from a solvent with polar groups capable of imparting to the solvent a high solvation power, but must not include those having labile hydrogen atoms.
U.S. Pat. No. 4,327,400, to Muranaka et al is directed to a double layer capacitor comprising a carbon electrode material with a polyvinyl pyrrolidone binder having a distinct feature of being very thin. The capacitor comprises two polarization electrodes and a separator therebetween. The electrodes are formed by carbon mixed with a binder applied to an expanded aluminum and the separator is impregnated with an electrolyte. The high ionic electrolyte is preferably obtained by mixing alkaline metal salts or ammonium salt and nitriles, sulfoxides, anides, pyrrolidones, carbonates, or lactones as a solvent.
U.S. Pat. No. 4,670,814, to Matsui et al is directed to a high-voltage capacitor constructed from a plurality of circular or flatly formed capacitor elements arranged in rows. The capacitor elements are insulated with synthetic resin before or at the time of molding the insulation layer. The reference is of interest in that it discloses a high-voltage bank of capacitor elements.